This invention relates generally to electrostatographic imaging and, more particularly, to electrostatographic compositions comprising toner particles surface-treated with a mixture of silica particles and particles of a metal salt of an aliphatic acid, and further to a process for forming the toner particles.
Digital electrostatographic printing products are being developed for printing high quality text and half tone images (cf. Schinichi Sata, et al., xe2x80x9cStudy on the Surface Properties of Polyester Color Toner,xe2x80x9d ISandT NIP13, 1997, pp 149-152, and Nash, R. and Muller, R. N. xe2x80x9cThe Effect of Toner and Carrier Composition on the Relationship Between Toner Charge to Mass Ratio and Toner Concentration,xe2x80x9d ISandT NIP 13, 1997, pp 112-120); thus there is a need to formulate electrostatographic toners and developers that produce improved image quality. Surface treatment of toners with fumed silica and/or titanium dioxide powders results in toner and developer formulations that have improved powder flow properties and reproduce text and half tone dots more uniformly without character voids (cf. Schinichi Sata, et. al., supra). The improved powder fluidity of the toner or developer can, however, produce unwanted print density in white background areas.
Over the xe2x80x9clifexe2x80x9d of an electrostatographic developer, its triboelectric charging characteristics change as prints are made. This instability in charging level is one of the factors that require active process control systems in electrostatographic printers to maintain consistent image-density from print to print.
There is a need in the art for developers that have improved stability and provide the advantages of improved electrostatic transfer and higher density capabilities.
In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, referred to as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they were formerly triboelectrically adhered by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator that comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially through the sleeve and serve to attract the developer composition to the sleeve outer surface, thereby forming what is commonly referred to in the art as a xe2x80x9cbrushed napxe2x80x9d. Either or both of the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060, 4,473,029 and 5,376,492, the disclosures of which are incorporated herein by reference in their entirety, teach the use of hard magnetic materials as carrier particles and also apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a xe2x80x9chardxe2x80x9d magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated, and an induced magnetic moment of at least 20 EMU/gm when in an applied magnetic field of 1000 Oersteds. When referring to magnetic materials, the terms xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d have the generally accepted meaning indicated on page 18 of B. D. Cullity, Introduction To Magnetic Materials, Addison-Wesley Publishing Company, 1972. Hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased, accompanied by good image development. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
In the methods taught by the foregoing patents, the developer is moved at essentially the same speed and direction as the electrostatic image to be developed by high speed rotation of the multi-pole magnetic core within the sleeve, the developer being disposed on the outer surface of the sleeve. Rapid pole transitions on the sleeve are mechanically resisted by the carrier because of its high coercivity. The toner particles disposed on the surface of the carrier particles that comprise the brushed nap of the carrier rapidly xe2x80x9cflipxe2x80x9d on the sleeve in order to align themselves with the magnetic field reversals imposed by the rotating magnetic core; and as a result, they move with the toner on the sleeve through the development zone in contact with or in close relationship to the electrostatic image on a photoconductor. This process is discussed in, for example, U.S. Pat. No. 4,531,832, the disclosure of which is incorporated herein by reference.
The rapid pole transitions, for example, as many as 600 per second on the sleeve surface when the magnetic core is rotated at a speed of 2000 rpm, create a highly energetic and vigorous movement of developer as it moves through the development zone. This vigorous action constantly recirculates the toner to the sleeve surface and then back to the outside of the nap to provide toner for development. This flipping action also results in a continuous feed of fresh toner particles to the image. As described in the above-mentioned patents, this method provides high density, high quality images at relatively high development speeds.
The direct interaction of the developer nap with the image member causes the developer to roll back toward the input side of the development zone. This rollback broadens the contact between the developer and the image member and thereby improves the development completion of the system.
The above-described development systems utilizing such hard magnetic carrier developers can have a tendency to display an increasing amount of xe2x80x9cdustingxe2x80x9d over time during use. The dusting phenomenon occurs when toner particles having a relatively low charge to mass (q/m) are literally flung off the developer nap of the rotating magnetic core toning roller. The rather violent chain flipping action characteristic of these development systems is believed to contribute to the dusting problem. While not wishing to be bound by theory, the mechanism thought to be responsible for such dusting is that the rate of charging of fresh replenisher toner decreases as a result of the loss of carrier charging ability by factors such as, for example, scum and fines buildup.
The present invention is directed to an electrostatographic toner composition that consists essentially of: noncrosslinked linear polymeric toner particles, about 0.7 wt. % to about 4 wt. % of hydrophobic silica particles disposed on the surface of the toner particles, and about 0.1 wt. % to about 2 wt. % of particles of a fatty acid metal salt disposed on the surface of the toner particles, wherein the weight percentages of the hydrophobic silica particles and the particles of a fatty acid metal salt are based on the weight of the polymeric toner particles. An electrostatographic developer is formed by mixing the toner composition so formed with hard magnetic carrier particles.
The present invention is further directed to a process for forming an electrostatographic toner composition that comprises: providing noncrosslinked linear polymeric toner particles of a selected particle size, and dry blending the polymeric toner particles with a mixture consisting essentially of about 0.7 wt. % to about 4 wt. % of hydrophobic silica particles and about 0.1 wt. % to about 2 wt. % of particles of a fatty acid metal salt, thereby forming a toner composition wherein the hydrophobic silica particles and particles of a fatty acid metal salt are disposed on the surface of the toner particles.